1. Field of the Invention
This invention relates to a marine propulsion unit for a watercraft, and more particularly to a hydraulic tilt and trim adjustment systern for a marine propulsion unit.
2. Description of Related Art
Stern drive units often propel both leisure and commercial boats. In general, owners and operators of leisure boats want easy and precise control over trim adjustment in order to maximize performance and handling characteristics of the boat.
The optimal trim angle of stern drive varies with a boat's running condition. For instance, the bow of the boat should press against the water when accelerating from rest or from a slow speed. To achieve this condition, the angle of the propeller shaft is disposed at a slightly negative angle relative to the horizontal (i.e., at a negative trim angle). A thrust vector produced by the propeller in this position is thus out of the water. When running at high speed, the propeller is raised or trimmed to position the propeller shaft at a positive trim angle relative to the horizontal within the range of about 0.degree. to 15.degree.. The outboard drive also must be raised beyond the normal trim range in order to operate in shallow water, to avoid underwater articles and for storage in a full tilt-up position.
In commercial applications, such as for fishery use, boat operators desire a quick tilt-up function for the stern drive. Fast tilt-up of the drive is required to rapidly place the drive's propeller out of the water to avoid nets and the like, which float on the water surface.
A hydraulic tilt and trim adjustment systern often adjusts the trim and tilt position of the stern drive. The tilt and trim adjustment systern usually includes at least one hydraulic actuator which essentially operates between the boat transom and the stern drive unit. The actuator causes the stern drive to pivot about a horizontal axis to raise or lower the drive's unit.
Tilt and trim adjustment systerns also usually employ a hydraulic motor that effects the trim and tilt operations of the outboard drive. For this purpose, prior hydraulic motors have included a reversible electric motor that selectively drives a reversible fluid pump. The pump pressurizes or depressurizes the hydraulic actuator for raising or lowering the stern drive.
The pressure in the actuator required to move the stern drive varies greatly depending on whether the propulsion unit is operating in a trim range or in a tilt range. In a tilt range, usually associated with tilting the propulsion unit out of the water, the pump generates a relatively low pressure in the actuator because only the drive unit loads the actuator.
The pump conversely must generate far greater pressure to trim-up the stern drive because of the load placed on the unit by the propulsion unit. The increase in load results from the thrust of the propulsion unit. That is, a portion of the thrust produced by the propulsion unit acts downward and against the tilt and trim mechanism when trimming up. Higher pressures therefore are required in the actuator to trim up the drive when running at high speeds (e.g., at planning speeds). When used with leisure boats (e.g., ski boats, sport boats, run-abouts, and the like), the tilt and trim adjustment systerns are designed to trim the outboard drive relatively slowly to prevent drive "pop-up."
Undesirable drive pop-up occurs because the thrust of the propulsion systern suddenly decreases as the motor is swung from the trim range into the tilt range. Within the tilt range, the large pressure built-up within the actuator, which was opposed by the drive's thrust in the trim range, rapidly pushes the actuator arm upward and causes the stern drive to pop-up quickly. Tilt and trim mechanisms used on leisure boats thus limit trim and tilt-up speed.
As noted above, however, it often is desirable in commercial applications to quickly raise the stern drive in order to avoid underwater articles, such as, for example, fishing nets and the like. The hydraulic circuitry employed with tilt and trim mechanisms used in commercial applications therefore permits the stern drive to be raised quickly.
Because of the differences in the design of the hydraulic circuitry, it previously has not been easy to convert a tilt and trim adjustment systern for commercial applications. Prior drive units also have not been designed to exhibit the trim and tilt adjustment characteristics necessary to make the drives acceptable for use in both leisure and commercial applications.
Prior attempts to provide such a versatile tilt and trim adjustment systern have suffered from several drawbacks. For instance, U.S. Pat. No. 3,842,789 discloses a valve systern which permits quick tilt and trim movement of an outboard drive unit; however, this systern requires the manual control of a remote operator in order to actuate the valve and quickly raise the outboard drive. Actuation of the valve occurs upon operation of a shift control mechanism. The control of the drive systern does not differentiate between movement within the tilt and trim ranges, only the drive direction (e.g., forward/reverse).
Other methods have been proposed to quicken the operational speed of the tilt and trim adjustment systern. One such approach involved reducing the cylinder diameter of the actuator. Although this approach quickened the tilt-up speed of the mechanism, it failed to maintain certain trim angles when running at high speeds or under large loads. It also did not provide the shock-absorbing function conventionally provided by prior hydraulic adjustment systern when the drive strikes a floating object.
Another approach involved increasing the fluid flow volume from the hydraulic pump. Although this may quicken the operational speed of the hydraulic systern, it requires almost all new parts, increasing either the costs associated with fabrication or retrofit.